Electron probe apparatus having an objective lens with an aperture for restricting fluid flow



Oct. 10, 1967 H. NEUHAUS 3,346,736

ELECTRON PROBE APPARATUS HAVING AN OBJECTIVE LENS WITH AN APERTUHEFORRESTRICTING FLUID FLOW Filed Sept. 22, 1964 ,0 (yr/w will My My llFIG. I

EXHAUST DIFFI PUMP [as FORE PUMP 94 m 20- J EXHAUST AUXILIARY PUMPEXHAUST/ 92 I ATTORNEY United States Patent ration of Delaware FiledSept. 22, 1964, Ser. No. 398,280 1 Claim. (Cl. 250-495) This inventionrelates to a novel instrument for rapid chemical analysis of selectedmicroscopic regions of a specimen by X-ray emission techniques, and,more particularly, to a novel arrangement for focusing an electron beamupon the specimen and for supporting the specimen.

Instruments of the type with which the invention is concerned are knownas microprobes, and generally include an electron gun, means forbringing the electron beam emitted by the gun to a line focus, means forsupporting a specimen to be analyzed with its surface at the focus ofthe electron beam, a microscope for viewing the surface of the specimenat the point where the electron beam impinges upon it, and one or moreX-ray spectrometers for measuring the X-rays of various difierentwavelengths emitted by the specimen in response to the electronirradiation. Instruments of this type are currently gaining wideacceptance in industry and are being found to be advantageous for use inmany difierent fields. Heretofore, however, they have been relativelyexpensive, and have not been well suited for analyzing materials thatcannot withstand exposure to a hard vacuum.

Accordingly, one important object of the present invention is to providea novel X-ray instrument for microanalysis, which is of relativelysimple and inexpensive construction.

Another object is to provide an instrument of this type including meansfor maintaining the electron beam system within a hard vacuum, whilesimultaneously allowing the specimen to be maintained in an atmosphereat appreciable pressure, thereby to permit analysis of materials thatcannot satisfactorily withstand exposure to a hard vacuum.

The foregoing and other objects and advantages of the invention willbecome apparent in the following detailed description of a presentlypreferred embodiment thereof, taken in conjunction with the drawing,wherein:

FIGURE 1 is a cross-sectional view of an X-ray analytical instrumentaccording to the invention, particularly showing the electron beamfocusing arrangement and manner of supporting the specimen.

FIGURE 2 is an exploded, perspective view of a specimen holder accordingto the invention, together with a specimen prepared to fit within theholder.

FIGURE 5 is a partly schematice plan view, partly in section,and on areduced scale of the instrument shown 1 in FIGURE 1.

Briefly, the invention contemplates an instrument in which the electronbeam focusing system is enclosed and sealed except for an exhaust portand an aperture through which the electron beam is emitted. The aperturefor the electron beam opens into an outer housing, which may bemaintained at a pressure greater than the hard vacuum required forprotection of the electron gun and for proper focusing of the electronbeam.

In operation, leakage of air or other gas into the inner enclosure isrestricted by a relatively small aperture at the objective electron lensto a value which is easily handled by the exhaust diifusion pump Withoutpermitting a significant increase in pressure in the inner enclosure.

The instrument does not require an internal stage for mounting andpositioning the specimen under examination. Instead, there is anaperture in the wall of the outer 3,346,736 Patented Get. 10, 1967housing in alignment with the electron beam, and the specimen is sealedagainst the outer surface of the outer housing for examination, eitherby itself or within a cup, the rim of which seals against the housingwall. The arrangement is simple and inexpensive, yet fully practical andhighly convenient.

A novel optical arrangement is also provided for illuminating and forviewing the specimen during analysis without interfering with theelectron beam or the X-ray detectors.

Referring now to the drawing, the instrument shown therein includes anelectron gun (not shown) mounted within an upper housing portion 10 andpositioned to.

direct an electron beam downwardly along the central vertical axis ofthe instrument. The beam passes first through a condenser lens 12, thenan objective lens 14, and finally impinges upon the specimen 16.

Although from one viewpoint, an electron lens consists of the magneticor electric field which acts upon electrons passing through it, it hasbecome common usage to refer to the physical parts used for producingthe field as the lens, and the term is used in this latter sense in thisapplication.

The two lenses 12 and 14 constitute the electron beam focussing system.They are fixed to a cylinder 18, which fits partly into the main housing20 of the instrument and supports the lenses therein. The condenser lens12 is secured in the cylinder 18 upon a ring 30, which rests upon aninwardly projecting shoulder 31 in the inner wall of the cylinder. Anouter flange 22 of the lens 12 rests upon the ring 30, and is firmlysecured thereto by a retainer ring, which overlies the lens and is screwattached to the ring 30. A tube 32 constituting a diaphragm aperture forthe electron beam is fitted within the bore 34 of the condenser lens forlimiting the beam.

The objective lens 14 is secured against the lower end of the tube 18 insealing engagement with an inner ring 36. The ring 36 is welded orbrazed to the inner wall of the cylinder 18, and the seal between thering 36 and the top of the objective lens 14 is accomplished by anO-ring 38. The objective lens 14 is retained in pressure engagementagainst the O-ring 38 by a retainer ring 40 which is screw attached tothe ring 36. A viewing window 41 is provided in the wall of the cylinder18 near its upper end for viewing a retractable fluorescent screen 43.The screen 43 is used during preliminary adjustment of the instrument,and is retracted out of the electron beam for normal operation.

The structure so far described constitutes the inner en clsoure assemblyof the instrument, and is removably fitted through an aperture 42 in thetop wall of the housing 24 A ring 44 is brazed or otherwise securedaround the outside of the cylinder 18 in sealing engagement with it, andan O-ring 46 is positioned to seal between the bottom surface of thering 44 and the top surface of the housing 20.

The cylinder 18 also fits sealingly through an aperture (not separatelydesignated) in a shelf 48 within the housing 20, thereby being held inproper vertical alignment within the housing. One or more ports 50 inthe cylinder 18 provide communication between the interior of thecylinder and the region 52 within the housing between the shelf 48 andthe top wall of the housing. In operation the region 52 is at the samepressure as the interior of the cylinder 18. Electrical leads 54 for thecoil of the objective lens 14 are trained through one of the ports 50around the edge of the condenser leans 12, thereby permitting the leadsfor both of the coils to be connected to a common jack 55 in the wall ofthe cylinder 18.

In order to permit maintenance of a substantial atmosphere in the outerhousing 20 while still maintaining a hard vacuum within the cylinder 18,a pair of diaphragm aperture plates 56 are fitted within the bore of theobjective lens 14 just above the magnetic gap '70. The bore of the gap,in the embodiment illustrated is 0.2" and the diameter of the aperturesin the plates 56 is 0.3 mm. The aperture plates 56 restrict the flow ofatmosphere so that a diffusion pump of conventional size, such as onerated at 300 liters per second at 10- mm. Hg pressure can still maintaina hard vacuum of less than mm. Hg when the pressure within the outerhousing is as high as about 100 microns Hg.

A second shelf 53 is brazed, or otherwise sealed across the housing 20just beneath the lower tip of the objective lens 14 to provide an airlock 5%, which may be vented to the ambient atmosphere without raisingthe pressure within the major part of the housing 20. A central aperture62 is provided in the lower shelf 58, and a gate valve 64 is arrangedfor selectively opening and closing the aperture 62. An aperture 66 isformed in the bottom wall of the housing 20 to permit the electron beamto pass through the wall and impinge upon the specimen 16 when the gatevalve 64 is open.

For economy in manufacture, the condenser lens 12 and the objective lens14 are preferably of substantially identical construction. The peculiarshape of the lenses is, however, dictated by considerations afiectingprincipally the objective lens 14. For optimum focussing of the electronbeam, the bombarded surface if the specimen 16 should be normal to thebeam, and in order to minimize the effects of self absorption, and ofanomalies and irregularities in the surface of the specimen 16, thetake-off angle should be high, that is, the X-rays selected fordetection should be those emitted by the specimen at as large an anglefrom the specimen surface as possible. For this purpose, the coilhousing of the objective lens 14 is extended downwardly and taperedtoward the specimen. This permits the detection of those X-rays thatpass upwardly from the surface of the specimen relatively close to theaxis of the electron beam. The tapered wall 72 is arranged to liegenerally along a cone having its apex at the focal point of theelectron beam, thereby permitting the achievement of the maximum takeoffangle consistent with the diameter of the cylindrical portion 74 of thecoil housing and the focal length of the lens. In the embodiment of theinvention illustrated herein, the focal length of the objective lens 14was about 1 /2", and the conical angle of the tapered portion 72 of thelens was about 60, permitting a take-off angle close to 60 from thespecimens surface.

The sample holding arrangement of the present invention does away withthe need to provide an expensive stage arrangement within the evacuatedhousing 24) for supporting the specimen. As illustrated, the specimen 16may be cut to size and placed Within a shielded cup '78, which is merelyplaced against the bottom of the housing 20 with an O-ring 8h interposedto seal it. The air lock 69 is evacuated. The gate valve 64 is opened,and the atmospheric pressure against the cut '78 is sufficient to holdthe cup and specimen in place against the bottom of the housing 20. Thespecimen then may be moved by hand when it is desired to analyzedifferent portions of its surface, or any desired cross slide (notshown) may be used to control its motion. Alternatively, the specimen16, itself, may be sealed against the bottom of the housing 20. Thisarrangement is especiaily useful when it is desired to work with largespecimens, or to avoid cutting a sample from a large workpiece.

Another important advantage of the specimen holding arrangement of theinvention it its ready adaptability to illumination of the specimen bylight transmitted through it, as is often desired, particularly ingeological work. For this purpose, a window 77 is provided in the bottomof the cup '78, and a bottom illuminator 79 is aimed at the specimenthrough the window. The illuminator 79 is not in the housing 20, but inthe ambient atmosphere, so it may be of any desired power and heatdissipation without the need of considering the special problems thatwould arise if it were within the evacuated housing.

The exhaust port 82 of the cylinder 18 is connected to the input of adiffusion pump 84 so that the electron gun and the electron focusingassembly is maintained at a desired hard vacuum pressure. The interiorof the main housing 20, however, outside of the inner assembly includedin the cylinder 18 may be maintained at any desired pressure from a hardvacuum up to about 100 microns Hg by adjustment of a modulatable valve86, which is connected between this exhaust port of the housing 20 andthe inputs of the diffusion pump 84 and an auxiliary forepump 89. It isbelieved that for many specimen materials, the preferred pressures willbe from a hard vacuum up to about one micron Hg.

In order to avoid unacceptable attenuation of X-rays emitted by lightelements, the pressure within the outer housing should be kept belowabout 100 microns Hg. Also, when high voltages are present within theouter housing 20, such as are used in certain types of X-ray detectors,it becomes necessary to maintain the pressure within the housing 2%below about 10 microns Hg.

Even with pressures as high as 100 microns Hg in the outer housing 20,the aperture plates 56 restrict the flow of atmosphere into the cylinder18 sufficiently to allow the diffusion pump 84 to maintain asatisfactory hard vacuum within the cylinder 18.

The optical system for illuminating and viewing the upper surface of thespecimen includes prism mounted with one of its faces against the bottomof the objective lens 14, and centrally apertured to permit the electronbeam to pass through it without interference. The other two faces 9%)and 90" of the prism are reflective. The illumination enters from theleft, as viewed in the drawing, and is reflected to the specimen by theleft hand face 90 of the prism, while the viewing optical axis is bentby the right hand face 90". A microscope 91 is sealed through the wallof the housing 20 aimed at a slight downward angle so that its projectedoptical axis is reflected by the prism 90 to intersect the electron beamat its focal point. The illuminator 92, similarly to the bottomilluminator 79 is situated outside of the housing 20, thus avoidingcooling problems and enabling the provision of high intensityillumination with convenience.

The optical system is not only arranged for the convenient use of highintensity illumination in either the reflective or transmission modes ofilluminating the specimen, but also permits optical viewingsimultaneously with the electron bombardment and the taking of X-raymeasurements. This is an important feature, which is lacking in many ofthe microprobe instruments heretofore known. Moreover, the opticalsystem of the present invention is extremely simple and inexpensive, yetflexible and highly satisfactory in use.

One or more spectrometers are arranged within the outer housing 20 fordetecting X-rays emitted by the specimen 16 responsively to electronbombardment. The spectrometer, or spectrometers 94, typically two asshown in FIGURE 3 may be of any desired type, and form no part of thepresent invention.

What is claimed is:

Apparatus for bombarding an object with an electron beam to cause theobject to emit X-rays, said apparatus comprising:

(1) an electron gun,

(2) a magnetic condenser lens,

(3) a magnetic objective lens,

(4) a tubular housing for mounting said gun and said lenses in coaxialalignment with each other, said objective lens being mounted at theopposite end 0f said housing from said gun and being sealed to saidhousing so that the sole opening at said opposite end is through themagnetic gap of said objective lens,

(5) an outer housing for receiving said tubular hous- 5 6 ing with amajor portion of said tubular housing (9) an X-ray spectrometer mountedin said outer houswithin said outer housing and with said objective lensing for detecting X-rays emitted by an object ex- Within said outerhousing and with said gun outside posed to the electron beam, and ofsaid outer housing, (10) an optical system for visually observing anobject (6) aperture means for restricting fluid flow through 5 exposedto the electron beam.

the magnetic gap of said objective lens, whereby in References Citedoperation a hard vacuum may be maintained within said tubular housingwhile a substantial pressure is UNITED STATES PATENTS present in saidouter housing, 2,241,432 5/ 1941 Von Ardenne et al. 25049.5 (7) meanssealing said tubular housing to said outer 10 2,899,556 8/1959 Schopperet al 250-49.5 housing, 2,944,172 7/ 1960 Opitz et al 25049.5 X (8)means for exposing an object to an electron beam 2,968,723 1/1961Steigerwald 250-495 passing through the magnetic gap of said objectiveOTHER REFERENCES lens out of said tubular housing, said exposing means nincluding an aperture through the exterior Wall of 15 The Encyclopedloof Y and Gamma Rays, edlted Said Outer housing aligned with the magneticgap by G. L. Clark, Reinhold Publishing Corp., New York, of saidObjective lens to allow the electron beam to July 1963: PP- 842 and passout of said outer housing and to impinge upon AL H G NILSON, PrimaryExaminer an ob'ect exteriorl th r of nd s al (1 t th exterirwan, y e e ae e agams e 20 W. F. LINDQUIST, Assistant Examiner.

